In U.S. Pat. No. 3,891,729, which is assigned to the same assignee as the present application, there is disclosed an apparatus for aerating a liquid in a container therefor. The apparatus includes a hollow, star-shaped, multi-vaned rotor arranged in the bottom region of the container for rotation about a vertical axis, with the hollow interior of the rotor being in communication at its lower horizontal face with one end of a gas feed line the other end of which (in the most usual situation) is open to the ambient atmosphere outside the container at a location generally above the surface of the body of liquid therein. The rotor, as viewed in its direction of rotation, is provided at the trailing sides or flanks of its outwardly directed vanes with a respective set of gas exit openings, so that, as the rotor revolves at relatively high speeds, air is aspirated into its interior. Through its rotation, the rotor transports liquid located in the respective spaces between the various adjacent vanes outwardly of the rotor by means of the vertical leading flanks of the vanes, with the leading flank of each vane making an acute angle with a radial plane passing through the tip of that vane, the liquid having entered the inter-vane spaces from above and below the rotor. The aspirated air leaves the rotor through its gas exit openings and is transported outwardly of the rotor together with the liquid. A stator surrounds the rotor, the stator being formed by an upper and a lower ring and at least twelve circumferentially spaced vertical guide plates oriented at respective acute angles to the radial direction. Gas and liquid are mixed in the angular inter-vane spaces of the rotor and in the flow channels of the stator defined by the guide plates. The so-formed gas-liquid mixture is transported outwardly of and away from the rotor into the body of liquid in the container.
Similar aeration devices exist, in which the inflow of the liquid to the rotor is from above only and the aspiration of the air or gas is also from above.
In order to enable the treatment facility to not only purify waste water by an oxidative decomposition of the organic substances contained therein, but also to remove the even then still remaining nitrogen from the waste water by means of an anaerobic after-treatment, the waste water must be carefully mixed so that the sludge formed therein will be maintained in suspension. However, inasmuch as in the case of an anaerobic after-treatment of the waste water, an intake of air (oxygen) through the surface of the body of liquid must be inhibited to the greatest possible extent, it is essential that, during such mixing of the waste water, the surface of the body of liquid be kept as calm and undisturbed as possible.
All of the above-described known apparatus are advantageously well suited for aerating a liquid, but not for effecting only a limited mixing of the liquid in the absence of a gas intake, i.e., with the gas feed line shut off. Since the pumping energy needed for effecting such a limited mixing is very great, however, the power demand for driving the rotor becomes very high and necessitates the provision of an oversized motor. The pumping energy could, of course, be adapted to the requisite mixing action by controlling the rotational speed of the rotor, but effecting such a control of the rotational speed is expensive. Over and above that it is difficult to achieve a satisfactory correlation between the aeration with a predetermined quantity of air and the required pumping energy for a limited mixing in a given size of liquid container.